A semiempirical simulation study of differential capacitance c(d) is presented for the case in which the formula for the macroscopic surface potential is known. Both the differential capacity and the surface potential are treated as unique functions of the potential determining ions (H+ ions). The effects of surface heterogeneity on the surface charge density curves sigma(0) = f(pH), and capacity curves c(d) = f(pH), as well as on the position of maximum capacitance are discussed. The "model" effects (influence of the model parameter on the results) are presented. (c) 2005 Elsevier Inc. All rights reserved.